In a gas turbine engine, inlet air is continuously compressed, mixed with fuel in an inflammable proportion, and then contacted with an ignition source to ignite the mixture that will then continue to burn. The heat energy thus released then flows in the combustion gases to a turbine where it is converted to rotary energy for driving equipment such as an electrical generator. The combustion gases are then exhausted to the atmosphere after exchanging some of their remaining heat to the incoming air provided from the compressor.
Quantities of air in excess of stoichiometric amounts are typically compressed and utilized to keep the combustor liner cool and dilute the combustor exhaust gases so as to avoid damage to the turbine nozzle and blades. Generally, primary sections of the combustor are operated near stoichiometric conditions that produce combustor gas temperatures up to approximately four thousand (4,000) degrees Fahrenheit. Further along the combustor, secondary air is admitted that raises the air-fuel ratio and lowers the gas temperatures so that the gases exiting the combustor are in the range of two thousand (2,000) degrees Fahrenheit.
It is well established that NOx formation is thermodynamically favored at high temperatures. Since the NOx formation reaction is so highly temperature dependent, decreasing the peak combustion temperature can provide an effective means of reducing NOx emissions from gas turbine engines, and so can limiting the residence time of the combustion products in the combustion zone. Operating the combustion process in a very lean condition (i.e., high excess air) is a known method of achieving lower temperatures and hence lower NOx emissions.
In a liquid fuel turbine system, the liquid fuel injector orifices or outlets are within the combustor and thus exposed to substantial heat. During normal operations, this does not present a problem since the flow of liquid fuel through the liquid fuel injector provides a cooling effect. Further, the propagation of combustion along with the flow of air serves to prevent undesirable overheating of the liquid fuel injectors. Once operation ceases, however, neither liquid fuel nor air flows through the liquid fuel injector. Consequently, residual heat in the combustor area can cause elevation of the temperature of the liquid fuel injectors.
In terms of the materials of which the liquid fuel injectors are constructed, this temperature elevation experienced upon cessation of operation does not present a problem. However, the presence of residual liquid fuel in the liquid fuel injector at such time can cause a coking problem. The liquid fuel is carbonaceous in nature and upon being heated will begin to undergo a destructive distillation reaction, producing a coke-like and/or tarry residue.
This tendency to deposit carbon on fuel flow passages when liquid fuel is exposed to hot surfaces inside a gas turbine (coking) can quickly build up and may become severe enough so as to restrict, or even completely block liquid fuel flow through the fuel injector passages. Because in small gas turbines the liquid fuel atomization is generally controlled by small orifices that are located in regions of high temperature, the coking problem is of particular importance. With generally small fuel passages and atomizers, the effects of coking are more pronounced in a small gas turbine and can lead to poor fuel flow distribution and poor atomization, resulting in increased emissions, reduced combustor performance, and reduced system life.
In general, liquid fuel systems are designed so that the liquid fuel will not be hot enough to coke prior to injection into the combustor or into the premixing section of a lean pre-vaporize premix (LPP) combustor. During a shut down procedure, any stagnant liquid fuel left in the fuel injectors or passages that experiences high enough temperatures will very rapidly coke and lead to the aforementioned problems. The general approach to remedy this 1problem has been to purge the liquid fuel system by utilizing the engine pressure to push the liquid fuel out of the liquid fuel system through the injectors and other fuel passages. This approach may be employed when the engine pressure is high enough to overcome the various flow restrictions of the liquid fuel system, but it also results in a known amount of liquid fuel being discarded to the atmosphere. Further, if the engine pressure is not high enough, the fuel injectors and passages may not be cleared of liquid fuel, thus leading to coking.
What is needed is a method of purging liquid fuel from the fuel injectors of a gas turbine combustor at shutdown before any portion of the fuel is transformed into a solid deposit by the residual heat in the combustor, and without discharging the purged liquid to the atmosphere.
In one aspect, the present invention provides a method of shutting down a turbine engine having a fuel line for conducting liquid fuel from a fuel supply to a combustor fuel injector, the method comprising shutting off the fuel supply and passing compressed gas through the fuel line to purge fuel from the fuel line and the fuel injector into the combustor.
In another aspect, the present invention provides a turbogenerator comprising a turbine; a combustor for combusting fuel and compressed air to generate hot gas to drive the turbine, the combustor including a fuel injector; a fuel line connected to the fuel injector to supply fuel to the combustor from a liquid fuel source; an electric generator rotationally coupled to the turbine to generate electric power; and a source of compressed gas selectively coupled to the fuel line for passing compressed gas through the fuel line after shutdown of the turbogenerator to purge fuel from the fuel line and the fuel injector into the combustor.
In a further aspect of the present invention, the purged fuel is combusted in the combustor, and an igniter in the combustor may be used to ignite the purged fuel in the combustor. In a yet further aspect of the invention, the compressed gas is regulated to control combustion of the purged fuel in the combustor, and the compressed gas pressure may be regulated in accordance with a predicted combustor pressure.